The present invention relates to a method for recovering Al vapor and Al2O from off gases generated during the production of aluminum by carbothermic reduction of alumina.
Carbothermic reduction of alumina with carbon is highly endothermic and requires temperatures in excess of 2050xc2x0 C. for the production of aluminum. The production of aluminum at these high temperatures is accompanied by evolution of CO and substantial amounts of Al vapor and Al2O. The Al components in the off-gas back-react exothermically with the evolved carbon monoxide as the gas temperature is lowered. Such back-reaction is highly exothermic and represents a very large potential loss of energy. Furthermore it gives rise to the formation of deposits of aluminum oxycarbide, which are sticky and tend to block up off-gas conduits.
In U.S. Pat No. 4,099,959 (Dewing et al.), it has been proposed to avoid these difficulties by reacting the Al vapor and the Al2O in the off-gas with carbon to form a non-sticky Al4C3 with simultaneous generation of heat energy for preheating the carbon feed. This is achieved by passing the off-gas through a bed of relatively large pieces of carbon which are essentially stationary in relation to each other. In such a system there is, however, a risk of accidental formation of aluminum oxycarbide which will cause cementing of the lumps of carbon to one another.
In U.S. Pat. No. 4,261,736 (Dewing et al.), these problems are addressed by contacting the off-gas containing Al vapor and Al2O with particulate carbon in a fluidized bed maintained at a temperature above the temperature at which sticky aluminum oxycarbide forms and where heated carbon enriched with Al4C3 is removed from the fluidized bed. When using carbon particles as taught in the ""736 patent the surface of each carbon particle will become covered by reaction products and the reaction rate will thus be reduced as the gas must penetrate the reaction product layer on each carbon particle in order to continue the reaction. Only a part of the carbon in the carbon particles will thus be reacted to Al4C3. Consequently, the consumption of carbon particles in the process of U.S. Pat. No. 4,261,736 will be very high and the efficiency of the reaction low.
The present invention relates to a method for recovering Al components from the off-gas produced during the carbothermic production of aluminum, where the off-gas comprises CO, Al vapor and aluminum suboxide. It will be appreciated that aluminum suboxide is typically referred to as Al2O, but that the ratio of aluminum to oxygen varies over a broad range; the term aluminum suboxide as used herein is intended to encompass Al2O and compounds having aluminum to oxygen ratios other than 2:1. According to the present method, the off-gas is supplied to an enclosed reactor kept at a temperature of more than 1950xc2x0 C. A continuous supply of one or more gaseous or liquid hydrocarbon compounds, such as CH3 shown in the FIGURE, are provided to the enclosed reactor.
The compounds generally have the formula CnHm where n is a number from 1 to 12 and m is a number from 2 to 26. Preferably, the formula will be CnH2n+2, as for example CH4, or C2H6, and includes naphtha, gasoline and fuel oil. The hydrocarbons crack to form finely dispersed solid carbon and hydrogen gas in the enclosed reactor. This carbon has a very small particle size, from about 20 micrometers to about 500 micrometers and a very high surface area, from about 1 m2/g to about 10 m2/g. The carbon thus produced then reacts with the Al-components in the off-gas to produce Al4C3. The present Al vapor/Al2O recovery methods provide for a fast reaction between carbon and the Al containing compounds in the off-gases; the invention further provides a method wherein the temperature in the reaction zone can be easily kept above the critical temperature at which aluminum oxycarbide will form. Moreover, the use of cracked hydrocarbons reduces the temperature at which aluminum carbide is formed.
According to a preferred embodiment methane; ethane; propane; butane, that is C4H10; acetylene, that is C2H2; ethylene, that is C2H4; propylene, that is C3H6; butylene, that is C4H8; or a mixture of two or more of these gases are used. Thus triple bonded and double bonded hydrocarbons are useful. Decomposition of triple bonded and double bonded hydrocarbons give finely dispersed carbon black. It is not known if they will perform any better than saturated hydrocarbons. Preferred liquid compounds include naphtha, gasoline and fuel oil. Thus, both aliphatic and aromatic hydrocarbons can be used; aliphatic hydrocarbons can be saturated or unsaturated. It is within the scope of the invention to use mixtures containing both liquid and gaseous hydrocarbons.
In a preferred embodiment, seed particles are added to the enclosed reactor. The finely dispersed carbon formed by cracking of the hydrocarbon compounds will be deposited, at least partly, on the surface of the seed particles. This provides a large surface area of carbon which is available for reaction with the Al components of the gas. The seed particles may be any inert particles such as Al2O3, carbonaceous particles, aluminum carbide particles or a mixture of such particles. The preferred particle size for the seed particles fed should be as small as practically possible, and will thus depend on the reactor configuration. With a fluidized bed they might be about 100 micrometer, whereas in a moving bed they might be 1 millimeter.
The present invention provides for the substantially continuous production of finely dispersed carbon in-situ in the enclosed reactor or other enclosed space, as the hydrocarbon enters the reactor and is subjected to the environment maintained therein. Fresh reactive carbon will thus be readily available for reaction with the Al components in the gas. This in-situ produced carbon is very reactive, and, as noted above, will at least partly deposit on the surface of the seed particles and will react very fast with the Al vapor and Al2O in the off-gas. Al vapor and Al2O are sometimes collectively referred to herein as xe2x80x9cAl-containing compoundsxe2x80x9d or xe2x80x9cAl componentsxe2x80x9d.
Since the cracking reaction of the hydrocarbon compounds is endothermic, the temperature in the enclosed reactor can be closely controlled by regulating the amount of hydrocarbon compounds supplied to the enclosed reactor. This is done by measuring and controlling the flow of hydrocarbons, for example, for a liquid hydrocarbon a mass flowmeter could be used and for a gaseous hydrocarbon, an orifice could be used.
The method of the present invention can be carried out in any reactor suited for gas/solid reaction such as fluidized bed reactors, spouted beds, moving beds, rotary kilns and columns filled with seed particles. One skilled in the art will appreciate that these reactors are widely commercially available. The term xe2x80x9cenclosed reactorxe2x80x9d encompasses any of these reactors.
If columns filled with seed particles are used, it is preferred that the seed particles be carbonaceous or a mixture of carbon and alumina particles. The hydrocarbon compounds are preferably supplied to the lower part of the column and most preferably the hydrocarbon compounds are supplied to more than one level of the column to ensure that carbon is produced in the whole volume of the column. The seed particles on which Al4C3 is deposited can be withdrawn from the bottom part of the column and fresh seed particles can be supplied at the top of the column.
As stated above, finely dispersed carbon and hydrogen gas are formed when the hydrocarbon compounds crack in the enclosed space. The hydrogen gas formed will dilute the gas phase and will reduce the temperature at which aluminum carbide will form from about 2010xc2x0 C. to about 1950xc2x0 C. The method of the present invention is therefore carried out at a lower temperature than art-reported methods. This will reduce the heat stress on the material in the enclosed space and make the process more easy to run. Further, it has been found that the temperature range wherein sticky aluminum oxycarbide is formed is more narrow when gaseous or liquid hydrocarbon compounds are used as the carbon source, thereby providing another advantage of the present methods. For example, when using methane as the hydrocarbon compound it has been found that the temperature range where aluminum oxycarbide is formed is between 1940xc2x0 C. and 1955xc2x0 C., which is favorable, compared to a calculated temperature range between 1970xc2x0 C. and 2020xc2x0 C. for the art reported methods. Accordingly, the formation of aluminum oxycarbide in the present methods will occur at much lower temperatures than in art-reported methods. In the method of this invention, the temperature in the enclosed reactor will be over 1955xc2x0 C., preferably from 1955xc2x0 C. to 2000xc2x0 C.
The hydrogen produced in the enclosed reactor by cracking of hydrocarbon compounds will not react with the Al compounds to any substantial degree. In addition, the hydrogen will add to the calorific value of the gas leaving the enclosed reactor. The heat energy can be recovered from the gas leaving the enclosed reactor by combustion of the gas to produce steam, which can be used to produce electric energy in a generator. The solid material discharged from the enclosed reactor, which at least partly consists of Al4C3, is recycled to the carbothermic smelting furnace.
The reactor gas from the enclosed reactor, containing CO, H2 and some unreacted Al vapor and Al2O, is subjected to rapid cooling, preferably by mixing the gas with carbon and Al2O3 in a separate vessel. The carbon and Al2O3 is thereby preheated while Al vapor and Al2O are converted to carbon and Al2O3; minor amounts of aluminum oxycarbide are also produced. The preheated carbon and Al2O3 containing some aluminum oxycarbide are used as a raw material feed to the carbothermic reduction furnace.
The CO and H2 remaining in the gas has a high energy content and can be used to produce steam, which can in turn be used to produce electric energy in a generator.